Acoustic panels are widely used in the building construction industry. Acoustical panels are used on ceilings, on walls, in theatres or concert halls to control sound. These products absorb sound by allowing the sound to penetrate to the interior of the product, where the sound energy is absorbed. Access to the interior of the product is provided in many ways. Openings in the surface of the product allow sound to enter the product interior. The composition of the product can be designed to be at least partially transparent to sound, allowing it to pass through the product surface. Other means of admitting sound energy to the product interior can be used as are known to an artisan of acoustical products. Once inside the product interior, the sound energy is at least partially converted into mechanical or thermal energy and dissipated.
Conventional acoustical panels are made by either a felting process or a casting process. Cast products are usually higher in weight and in strength. These articles are made by mixing a fiber, binder with one or more fillers and additives and then placing the resulting pulp into trays or a mold for drying. The fiber, binder and the fillers are selected such that the resulting panel is sound absorbent. If additional sound absorbency is desired, additional voids or cavities are optionally formed into the surface of the product by embossing, pressing a design in the product. Another method of improving sound absorbency is by puncturing the product surface with needles (i.e., perfing). As with embossing above, perfing creates cavities that further promote sound absorption. One method of reducing the weight of cast products is by the addition of light weight fillers, such as expanded perlite. However, the perlite reflects sound and reduces the ability of the article to absorb sound. This results in balancing weight with sound absorbency to determine the amount of expanded perlite that is useful in a particular product.
An example of a product made by casting is described in U.S. Pat. No. 1,769,519. A pulp of mineral wool and a starch gel is cast into a mold and dried to form articles. Although these articles are effective in acoustical control, the articles are heavy. As a result, they are expensive to ship and lead to rapid fatigue in those who must transfer, carry or install the articles.
U.S. Pat. No. 4,655,950 reveals the use of an enzyme-modified, soy-based foaming agent to make a cast acoustical article that is lighter in weight. This patent teaches the addition of the foaming agent to the starch and water prior to thickening. The foamed pulp was then distributed on trays. The surface of the articles was screeded to impart a surface texture to the articles, then the wet pulp was dried in ovens at about 300° F. (149° C.) for about 16 hours.
Recent research has shown that the addition of the foaming agent prior to forming the starch gel does not always result in generation of a smooth, foamy pulp. In some circumstances, when the mineral wool fibers are added, the wool wads up into large knots, sometimes requiring shut down of the mixing equipment until the knots could be removed. The resulting pulp contained large lumps that were not aesthetically pleasing and also necessitated stopping the equipment, increasing the cost of the products.
Additional research indicates that surfactants are not consistently effective in producing a light weight acoustical product. As will be shown below, the addition of some surfactants actually caused an increase in product density. Creation of a lightweight cast article is not merely a matter of adding a surfactant and mixing the pulp to create a foamed article.
Thus, there is a long-felt need in the art for a method of making a lightweight cast article having acoustical properties. A need also exists for an acoustical product and method of making it that is free of expanded perlite. Further, a method of making an acoustical article is needed that avoids formation of large knots of mineral wool.